1. Field of the Invention
The present invention relates to an optical recording medium, in which writing, reading and/or erasing of information is conducted by utilizing a light beam or, more specifically, to an optical recording medium comprising a recording layer formed on a substrate in which a laser beam is applied to the recording layer from the side of the recording layer without passing through the substrate.
2. Description of the Related Art
The most optical recording media are the substrate side light incident type in which a laser beam is applied to the recording layer through a transparent substrate. Recently, however, optical recording media of the recording layer side light incident type, in which a laser beam is applied from the side of the deposited layers without passing through the substrate, have attracted attention as described in the magazine "Electronics" vol. 41, No. 5, May 1996, pp87-91.
Investigation of the recording layer side light incident type optical recording medium has begun because it has a higher recording density than the conventional substrate side light incident type optical recording medium. However, practical recording and reproducing using this type recording medium has not been reported. Accordingly, it appears that various problems must be solved before this type of medium is available for practical use.
Magneto-optical recording media and phase change-type recording media would be such possible optical recording media.
In a magneto-optical recording medium, light, particularly a laser beam, is applied to a recording layer to raise the temperature of the recording layer and reduce the coercive force of the recording layer, while the magnetization direction at the portion where light is applied changes by an external magnetic field to record or erase information. The temperature of the recording layer reaches about 200.degree. C.
A phase change-type recording medium utilizes a reversible structural change or a phase change between amorphous and crystal states of a material made by application of light, particularly a laser beam, for recording or erasing information. The temperature of the recording layer reaches about 600.degree. C. when recording and to about 170.degree. C. when erasing. The phase change-type recording medium has an advantages of high speed information processing and a high recording capacity. The phase change-type recording medium also has a merit of a low cost since the structure of a driver therefor is simpler than the driver for the magneto-optical recording medium.
The typical structure of these optical recording media comprises a stack of inorganic layers deposited on a transparent disc of polycarbonate by evaporation or sputtering. Commercial discs usually have a structure of polycarbonate (PC) substrate/lower dielectric layer/recording layer/upper dielectric layer/reflecting layer/organic protecting layer. The recording layer of the magneto-optical recording medium is made of a rare earth and transitional metal alloy such as TbFeCo and the recording layer of the phase change-type recording medium is made of a carcogen alloy such as AgInSbTe or GeSbTe. The dielectric layer of the magneto-optical recording medium is made of a nitride such as Si.sub.3 N4 and the dielectric layer of the phase change-type recording medium is made of a ZnS-based material such as ZnS.cndot.SiO.sub.2.
The phase change-type recording medium typically utilizes a crystal state of a recording layer as the erased state of information and an amorphous state of the recording layer, formed through fusion and rapid cooling using a high power laser beam, as the recorded state of information. The recording layer of a phase-change-type recording medium is amorphous when it is originally deposited, which is then entirely annealed to convert to a crystal state, i.e., the erased state, before it is used for recording. The entire annealing is conducted by using a laser beam, for example, with a spot of a width of 1-3 .mu.m and a length of about 100 .mu.m and with a power of about 1 Watt. This step of the entire annealing is called the initialization or initial crystallization.
In contrast to the above commercial typical optical discs, the recording layer side light incident type optical recording media, which are being investigated, would have a structure which has the order of the stack of the layers is reversed from that of the conventional one. That is, the recording media have a structure of PC substrate/reflecting layer/lower dielectric layer/recording layer/upper dielectric layer.